This application is based on Patent Application No. JP2000-143124 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an improved micro pump, and specifically relates to a micro pump for transporting minute amounts of fluid with high accuracy.
2. Description of the Related Art
The principal methods used by micro pumps to transport minute amounts of fluids include a first method using a mechanical check valve, and a second method using, in place of the check valve, a nozzle having different flow pass resistances in accordance with the fluid flow directions. A micro pump using the first method is disclosed in Japanese Laid-Open Patent Application No. HEI 11-257233, wherein a fluid is pressurized within the pump by operating a diaphragm, and this pressure is used to operate a check valve to transport the fluid. Japanese Laid-Open Patent Application No. HEI 10-299659 discloses a micro pump provided with movable valves in a nozzle unit communicating with a pressure chamber, wherein a piezoelectric element is used to open and close each of the movable valves to provide directionality to the flow of the fluid.
Japanese Laid-Open Patent Application No. HEI 10-110681 discloses a micro pump using the second method provided with projecting members in a nozzle unit communicating with a pressure chamber so as to have different flow pass resistances depending on the directions of the flow. This micro pump makes it difficult for fluid to start flowing in the opposite direction to a desired flow direction, such that the fluid is transported in one desired direction.
Since micro pumps using the first method are provided with check valves or movable valves, such micro pumps are mechanically complex, and readily susceptible to mechanical deterioration. Furthermore, the micro pump disclosed in Japanese Laid-Open Patent Application No. HEI 10-299659 requires at least three piezoelectric elements, including piezoelectric elements to operate the movable valves, and a piezoelectric element to change the pressure of the pressure chamber. A further disadvantage arises in that as these piezoelectric elements are operated individually, the drive circuits are complex.
Micro pumps using the second method can only transport a fluid in a single direction.
An object of the present invention is to provide an improved micro pump to eliminate the previously described disadvantages. More specifically, the present invention provides a micro pump which is capable of transporting minute amounts of fluid in both forward and reverse directions with high accuracy using a simple construction.
These and other objects are attained by one aspect of the present invention providing a micro pump comprising a first flow pass which changes flow pass resistance in accordance with a differential pressure, a second flow pass wherein the percentage change in the flow pass resistance corresponding to a differential pressure is smaller than that of the first flow pass, a pressure chamber connected to the first flow pass and the second flow pass, and an actuator for changing the pressure force within the pressure chamber. The differential pressure referred to herein is the pressure force at bilateral ends of a flow pass.
According to this aspect, the first flow pass has a resistance which changes in accordance with a differential pressure, and the percentage change in the resistance of the second flow pass corresponding to the differential pressure is smaller than that of the first flow pass. Accordingly, the ratio of the resistance of the first flow pass to the resistance of the second flow pass is different when the differential pressure is large and when the differential pressure is small. Since the actuator changes the pressure force within the pressure chamber connected to the first flow pass and the second flow pass, the ratio of the flow pass resistance of the first flow pass to the flow path resistance of the second flow pass can differ by changing the pressure within the pressure chamber. Therefore, a micro pump is provided which is capable of transporting minute amounts of fluid in forward and reverse directions with high accuracy using a simple construction.
It is desirable that the first flow pass and the second flow pass of the micro pump respectively have uniform cross sectional configurations taken in a plane that is orthogonal to the flow direction, and that the ratio of the cross sectional area to the flow pass length of the first flow pass is greater than the ratio of the cross sectional area to the flow pass length of the second flow pass.
According to this aspect, the ratio of the flow pass resistance of the first flow pass to the flow pass resistance of the second flow pass can differ when the differential pressure is large and when the differential pressure is small, since the first flow pass and the second flow pass respectively have uniform cross sectional configurations taken in a plane that is orthogonal to the flow direction such that the ratio of the cross sectional area to the flow pass length of the first flow pass is greater than the ratio of the cross sectional area to the flow pass length of the second flow pass.
It is further desirable that the first flow pass of the micro pump has any shape among a shape which rapidly changes cross sectional configurations taken in a plane that is orthogonal to the flow direction, a shape in which the center line is not straight, or a shape having an obstruction in the flow pass.
According to this aspect, the percentage change in the flow pass resistance relative to the change in differential pressure of the first flow pass is greater than that of the second flow pass since the first flow has any shape among a shape which rapidly changes cross sectional configurations taken in a plane that is orthogonal to the flow direction, a shape in which the center line is not straight, or a shape having an obstruction in the flow pass.
It is desirable that the micro pump is provided with drive means for driving the actuator to repeatedly change the volume of the pressure chamber between a first volume and a second volume at specific intervals, and this repetition is such that the time period when increasing the volume of the pressure chamber and the time period when decreasing the volume of the pressure chamber are different.
According to this aspect, the drive means drives the actuator to repeatedly change the volume of the pressure chamber between the volume of the first flow pass and the volume of the second flow pass at specific intervals. Since the time period of increasing the volume of the pressure chamber and the time period of decreasing the volume of the pressure chamber differ in this repetition, the differential pressures of the first flow pass and the second flow pass are different when the volume is increasing and when the volume is decreasing. As a result, the structure of the actuator may be simplified.
It is desirable that the driving means of the micro pump is capable of a first repetition and a second repetition wherein the time periods for increasing the volume of the pressure chamber differ.
According to this aspect, the direction of transport of the fluid in the first repetition is different from that of the second repetition because the time periods for increasing the volume of the pressure chamber are different in the first repetition and the second repetition.
It is desirable that the micro pump is provided with a drive means for driving an actuator to repeatedly change the volume of a pressure chamber between a first volume and a second volume at specific intervals, and the first flow pass has a flow pass resistance in a first direction which is greater than its flow pass resistance in a second direction opposite to the first direction, such that the drive means is capable of driving in a first repetition wherein the time period of increasing the volume is identical to the time period of decreasing the volume, and a second repetition wherein the time period of increasing the volume is different from the time period of decreasing the volume.
According to this aspect, the drive means drives the actuator to repeatedly change the volume of the pressure chamber between the volume of the first flow pass and the volume of the second flow pass at specific intervals. Since the first flow pass has a flow pass resistance in a first direction which is greater than its flow pass resistance in a second direction opposite to the first direction, a fluid is transported in the second direction in the first repetition wherein the time period of increasing the volume is identical to the time period of decreasing the volume, and a fluid is transported in the first direction in the second repetition wherein the time period of increasing the volume differs from the time period of decreasing the volume. Therefore, fluid can be effectively transported in both a forward direction and a reverse direction.